The Collaborative International Dictionary of English v.0.48:

Garrison \Gar"ri*son\, n. [OE. garnisoun, F. garnison garrison,
   in OF. & OE. also, provision, munitions, from garnir to
   garnish. See Garnish.] (Mil.)
   (a) A body of troops stationed in a fort or fortified town.
   (b) A fortified place, in which troops are quartered for its
       security.
       [1913 Webster]

   In garrison, in the condition of a garrison; doing duty in
      a fort or as one of a garrison.
      [1913 Webster]